1) Field of the Invention
The present invention relates to an oscillator employing a strip line of tri-plate structure as a resonant element.
2) Description of the Prior Art
FIG. 1 is an equivalent circuit diagram of a Colpitts oscillator. This type of oscillator is generally used in a mobile communication device.
In FIG. 1, capacitors C1 and C2 and a resonant element L constitute a positive feedback circuit for a transistor TR.
A strip line of tri-plate structure is often used as the resonant element L in the above Colpitts oscillator. The strip line of tri-plate structure comprises a substrate having a pair of grounding electrodes and a strip line electrode disposed between the pair of grounding electrodes. The strip line electrode acts as a resonant coil and resonant capacitors are formed between the strip line electrode and the grounding electrodes.
An oscillation frequency of the above oscillator is determined by a length of the strip line electrode in terms of design. However, the oscillation frequency is also influenced by other factors such as dispersion of the capacitances of the capacitors C1 and C2. Therefore, it has been mostly desired to eliminate or at least reduce such dispersion of the oscillation frequency due to the dispersion of the capacitances in the manufacturing process of the oscillator.
Generally, if capacitors having less dispersion of capacitances are employed, the dispersion of the oscillation frequency can be prevented. However, such capacitors cost too much and therefore, are not suitable for an oscillator which is compact, light and inexpensive. It is also conceivable to connect a variable capacitor in parallel with the resonant element L and vary the capacitance of the variable capacitor, thereby adjusting the oscillation frequency. In this case, although the costs of the capacitors C1 and C2 are not increased, the variable capacitor itself is expensive. Therefore, this also is not suitable for the above oscillator.